Ancient pottery reveals the past of Earth’s magnetic field.

About 3,000 years ago, a potter near Jerusalem made a big jar. It was meant to hold olive oil or wine or something else valuable enough to send to the king as a tax payment. The jar’s handles were stamped with a royal seal, and the pot went into the kiln.

Over the next 600 years, despite wars destructive enough to raze cities, potters in the area kept making ceramic tax jars, each one stamped with whatever seal represented the ruler du jour.

They didn’t know it, but in the process, the ancient potters were not just upholding centuries of tax bureaucracy.

As a group of archaeologists and geophysicists wrote Monday in the journal PNAS, they were also creating a long-lasting record of activity some 2,000 miles beneath their feet. And that record is now yielding clues to a big mystery about this planet: how its magnetic field has changed over time.

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“We are losing the magnetic field,” [Tel Aviv University archaeologist Erez] Ben-Yosef says. “We already lost more than 10 percent of its strength, so people are concerned that we might lose the magnetic field entirely.”

That’s concerning, he says, because the magnetic field protects the Earth from high levels of radiation coming from space.

But fear not, Ben-Yosef says. The jars hold hope.

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When Ben-Yosef and his colleagues studied 67 jar handles spanning from the late 8th century B.C. to the late 2nd century B.C., they found that the Earth’s magnetic activity has been a lot choppier than people expected.

For example, the jars indicate that in the late 8th century B.C., the core went a little crazy. The intensity of the magnetic field spiked to about double what it is today.

“It was the strongest it’s been, at least in the last 100,000 years, but maybe ever. We call this phenomenon the Iron Age spike,” Ben-Yosef says.

Then, it weakened quickly after 732 B.C.E., losing about 30 percent of its intensity in just 30 years.

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Geologist Steven Forman of Baylor University has also found evidence of a magnetic spike about 3,000 years ago, based on his study of Hall’s Cave in Texas.

“But we didn’t have the type of time resolution that the study in PNAS has,” he says, because it’s a lot harder to pinpoint rocks on a timeline than it is to pinpoint man-made objects. “That’s what so cool about what they did. They pulled this out of heated ceramics.”

He says the jar record shows the magnetic field is likely a lot more dynamic and nimble than researchers thought it would be.

Something to Believe In

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"Is it a fact—or have I dreamt it—that, by means of electricity, the world of matter has become a great nerve, vibrating thousands of miles in a breathless point of time?"— Nathaniel Hawthorne, The House of the Seven Gables, 1851